a. a middle aged mentally retarded person.
b. a person who is stupid, obtuse, or ineffective in most ways: socially inept, asks ridiculous or irrelevant questions, requires guidance in the most simplistic of tasks.
(b) I can't believe I had to explain myself four times! Wat a leetard.
"I was fine until some leetard sent me a text message that reminded me of the appalling state of our failing education system."